1. Field of the Invention
This invention relates to a system for computing the rotating speed of a rotating body, and more particularly it pertains to a system for computing the rotating speed of a rotating body, suitable for use in the computation of wheel speed of a motor vehicle in anti-lock control to prevent the wheels of the motor vehicle from being locked when brake is applied or in traction control to prevent the driving wheels of the motor vehicle from skidding when the motor vehicle is started or accelerated.
b 2. Description of the Prior Art:
In anti-lock control to prevent wheels of a motor vehicle from being locked when brake is applied, in traction control to prevent the driving wheels of the motor vehicle from skidding when the motor vehicle is started or accelerated, or in cruising control to permit a motor vehicle to run at a constant speed, computation of the wheel speed constitutes one of the most important factors. As is known in the art, accuracy of such control is greatly influenced by the accuracy in the computation of the wheel speed and the time required for such computation. Especially in anti-lock control and traction control, it is required that the wheel speed changing every moment be accurately detected; thus, highly accurate sensors should be employed to achieve such accurate detection of the wheel speed.
Such a rotor may comprise a toothed rotor called as tone wheel rotating with the wheel, and a pickup coil wound on a permanent magnet provided in the neighborhood of the rotor. As the tone wheel rotates, the distance between the tone wheel and the permanent magnet varies with the position of the teeth of the tone wheel, and as a result, the pickup coil generates an a.c. voltage with a frequency proportional to the rotating speed of the tone wheel. To compute the wheel speed from this a. c. voltage, the sinusoidal waveform of the a. c. voltage is shaped into a square waveform so that a pulse train signal such as shown in FIG. 1 is obtained. Reference time periods T of a predetermined length for speed computation are set in succession, and measurement is made of the time length from the first rising edge to the next rising edge of pulse train signal in each speed computation, i.e., time period t of one cycle (this will be referred to as "single-edge detection" hereinafter). Wheel speed Vw is calculated from the time period t using the following equation: EQU Vw=1/(t.times.k)
where k is a constant.
To avoid an impossibility of speed detection which tends to be caused when the frequency of the sinusoidal waveform derived from the pickup coil is too high while the wheels are running at high speed, it is the usual practice that a relatively small number of teeth is selected for the tome wheel. Thus, as will be seen from FIG. 2, when the single-edge detection as described above is made while the wheels are rotating at low speed, it tends to happen that measuring time period t extends to go beyond the range of speed computation reference time T, and if that happens, speed computation becomes impossible with respect to the respective speed computation reference time period T, thus resulting in inaccurate speed computation and deteriorating the control performance of anti-lock control and so forth. For example, if the number of teeth of the tone wheel is selected such that the pickup coil generates an a.c. voltage of 15 Hz per 1 km/h and the length of the speed computation reference time T is set at 8 ms, then it follows that measuring time t extends to go beyond the range of the speed computation reference time T when the frequency of the a.c. voltage signal derived from the pickup coil is 250 Hz or less (16.6 km/h in terms of wheel speed).